You absolutely cannot build a fission bomb with uranium enriched to only 20% without further enrichment. I don't know who's saying that but they're full of shit. You need uranium enriched to 80% at a minimum and even that requires a very clever bomb design and exotic neutron sources to supplement the uranium. Most uranium bombs require 90-95% enrichment to work and even then resist miniaturization.
This paper isn't actually about US SMRs. It's a stalking horse for claiming that the Iranian nuclear program, which halts enrichment of uranium at 20%, is a proliferation risk in its current form rather than requiring significant re-enrichment of its stocks that would be rather easily detectable under the framework of the 2014 E3+3 deal.
Except... Iran hasn't halted uranium enrichment at 20%. Try 60%. Source: International Atomic Energy Agency report dated 26 Dec 2023. "In a Dec. 26 report, the IAEA noted that Iran is now producing approximately nine kilograms of uranium enriched to 60 percent uranium-235 per month. Iran was producing 60 percent enriched U-235 at a similar rate in early 2023, but decreased production by about two-thirds in June. (See ACT, October 2023.) Accelerating the production of uranium enriched to 60 percent U-235 is concerning because the material can be quickly enriched to weapons-grade levels or 90 percent." https://www.armscontrol.org/ac...) If you really believe Iran is only pursuing "peaceful nuclear energy solutions" I have a bridge to sell you, as well as some swamp land in Florida.
At the source of every error which is blamed on the computer you will find at least two human errors, including the error of blaming it on the computer.